1. Field of the Invention
The present invention relates generally to thrust bearings, and more particularly to a static pressure thrust bearing interposed between a pump shoe and a swash plate of an axial piston type hydraulic pump and motor for use within vehicles.
2. Description of the Prior Art
Conventional axial piston type hydraulic pumps and motors comprise a revolving cylinder barrel having a plurality of parallel cylinders disposed therein and pistons slidably fitted within the cylinders and movable therein by means of a swash plate, each piston having a convex surface end which is swaged within a piston shoe having a peripheral land portion and a static pressure pocket. A smooth surface is usually provided upon the peripheral land portion so as to facilitate the sliding movement of the piston and cylinder apparatus over the surface of the swash plate.
Operating oil, which is also used as lubricating oil, passes axially through the pistons, and the static pockets provided within the piston shoes normally contain very small particulates, such as for example, metallic powder particles which are normally continuously being produced during operation of the movable parts, floating dust within the air and mixed into the lubricating oil, and the like. Accordingly, both the sliding surfaces of the peripheral land portion and the swash plate tend to be injured or scratched whereby gaps may be formed thereon during the operation of the pump motor. Lubricating oil leakage is then increased through the gaps and consequently the efficiency and durability of the hydraulic pump and motor substantially decreases due to the increase in friction between the sliding surfaces as a result of the lubricant loss, and consequently, the sliding surfaces between the peripheral land the swash plate become over-heated and excessively worn.